1. Field of the Invention
This invention relates to heat recuperators for high temperature combustion furnaces.
2. The Prior Art
High temperature combustion furnace operations particularly those operating above about 1000.degree. C consume enormous quantities of energy. Such high temperature furnace operations include, for example, copper smelters, glass furnaces, steel furnaces, and the like. Customarily, the combustion furnace is constructed as an enclosed, heat-resistant vessel containing a pool of material being heated. A combustion flame is directed into the enclosure over the top of the material being heated. Customarily, only about 15-25 percent of the thermal energy of the flame is absorbed by the material, the remaining 75-85 percent of the thermal energy is lost. About 30 percent of the thermal energy lost is lost through the furnace walls, roof, floor and exhaust duct work and is carried out with the exhaust. This thermal energy loss represents a significant quantity of energy that must be supplied by consumption of additional fuel unless some form of energy recuperation is practiced.
The portion of thermal energy discharged to the atmosphere as exhaust represents a significant quantity of potentially recoverable thermal energy. However, as a result of the very high temperatures and, occasionally, the corrosive environment encountered in the exhaust gases, very few structural materials can successfully withstand prolonged exposure to the hot exhaust gases.
One conventional heat recovery technique involves directing the hot exhaust gases through a chamber containing a grid work of refractory bricks known as a checker system. The checker system is formed as two, separate systems so that hot exhaust gases can be diverted through the bricks in one system until they are heated to an optimum temperature. The exhaust stream is then switched into the second system while incoming air is drawn through the first system and heated by absorbing thermal energy from the heated bricks in the first system. The exhaust and airstreams are alternately switched between the two checker systems at set intervals of about 20-30 minutes. The result is that the incoming airstream to the combustion furnace does not have a constant temperature but a cyclically varying temperature. This results in loss of furnace efficiency and difficulty in accurately controlling the thermal energy input to the furnace and also results in loss of efficiency because of the difficulty in controlling the air/fuel ratios.
Additionally, checker systems occupy a large space and involve relatively elaborate duct work and valving systems thereby requiring relatively high initial construction cost and ongoing maintenance costs.
Attempts to avoid the problem associated with the checker system of heat recuperation has led to the use of lower temperature heat recuperation systems. For example, a typical glass furnace operates at a relatively high temperature (approximately 1000.degree.-1650.degree. C) which means that the exhaust gases therefrom would be far in excess of the maximum temperature capabilities of most metals. Accordingly, it is conventional to dilute the hot exhaust with outside air and, thereby, lower the exhaust temperature so that a standard metal recuperator can be used. However, dilution causes a tremendous loss in the enthalpy of the exhaust stream and, consequently, a tremendous loss in the recuperator efficiency.
Furthermore, many exhaust systems carry fumes that are extremely corrosive to most metals. It has also been found that fumes carried over with the exhaust stream tend to condense on the cooler recuperator surfaces. At high temperatures, the condensate tends to be a corrosive fluid while at lower temperatures the fumes crystallize as a dust having a fuzzy, crystalline characteristic which tends to form an insulative layer in the exhaust duct work. This layer must be removed periodically so as to enhance heat transfer and lower the resistance to flow of the hot exhaust gases.
Cleaning of large heat recuperators is difficult, time consuming and, therefore, expensive unless the recuperator may be readily disassembled and reassembled from elements which are easily handled and cleaned.
An energy balance between the hot exhaust gases and the incoming airstream shows that for maximum efficiency a greater quantity of air can be heated than is used for supporting combustion. Accordingly, it would be advantageous to divert a portion of the heated air as a heat source for a lower temperature process such as an annealing furnace or the like. However, the cyclically varying temperatures resulting from an airstream passed through a conventional checker system would render the heated airstream unfit for use in any annealing furnace requiring a reasonably controlled temperature.
It is also desirable to divert a portion of the heated air as an auxiliary heated airstream for use in structure heating as comfort conditioning. However, when used for comfort conditioning, great care must be exercised to insure that combustion products are specifically precluded from entering the comfort conditioning system. Accordingly, it is usually conventional to make no attempt to use any of the recovered heat for comfort conditioning.
It would, therefore, be a significant advancement in the art to provide a high temperature heat recuperator apparatus and method whereby the heat recuperator is readily fabricated from a plurality of standardized, interchangeable, modular elements. It would also be an advancement in the art to provide a recuperator wherein the modular elements in contact with the most destructive portion of the exhaust gases are fabricated from a highly refractory ceramic material while the remaining portion of the recuperator modules may be, selectively, inexpensively fabricated from conventional metallic materials. Another advancement in the art would be to provide a heat recuperator wherein the incoming airstream is pressurized so as to inhibit the infiltration of exhaust gases into the airstream and thereby accommodate diverting at least a portion of the airstream for use in auxiliary heat systems and, more particularly, for use in comfort conditioning. Such an invention is disclosed herein.